Distinct Fcα receptor N-glycans modulate the binding affinity to immunoglobulin A (IgA) antibodies.

Human immunoglobulin A (IgA) is the most prevalent antibody class at mucosal sites with an important role in mucosal defense. Little is known about the impact of N-glycan modifications of IgA1 and IgA2 on binding to the Fcα receptor (FcαRI), which is also heavily glycosylated at its extracellular domain. Here, we transiently expressed human epidermal growth factor receptor 2 (HER2)-binding monomeric IgA1, IgA2m(1), and IgA2m(2) variants in Nicotiana benthamiana ΔXT/FT plants lacking the enzymes responsible for generating nonhuman N-glycan structures. By coinfiltrating IgA with the respective glycan-modifying enzymes, we generated IgA carrying distinct homogenous N-glycans. We demonstrate that distinctly different N-glycan profiles did not influence antigen binding or the overall structure and integrity of the IgA antibodies but did affect their thermal stability. Using size-exclusion chromatography, differential scanning and isothermal titration calorimetry, surface plasmon resonance spectroscopy, and molecular modeling, we probed distinct IgA1 and IgA2 glycoforms for binding to four different FcαRI glycoforms and investigated the thermodynamics and kinetics of complex formation. Our results suggest that different N-glycans on the receptor significantly contribute to binding affinities for its cognate ligand. We also noted that full-length IgA and FcαRI form a mixture of 1:1 and 1:2 complexes tending toward a 1:1 stoichiometry due to different IgA tailpiece conformations that make it less likely that both binding sites are simultaneously occupied. In conclusion, N-glycans of human IgA do not affect its structure and integrity but its thermal stability, and FcαRI N-glycans significantly modulate binding affinity to IgA.

An NMR and MD study of complexes of bacteriophage lambda lysozyme with tetra- and hexa-N-acetylchitohexaose.

The X-ray structure of lysozyme from bacteriophage lambda (λ lysozyme) in complex with the inhibitor hexa-N-acetylchitohexaose (NAG6) (PDB: 3D3D) has been reported previously showing sugar units from two molecules of NAG6 bound in the active site. One NAG6 is bound with four sugar units in the ABCD sites and the other with two sugar units in the E'F' sites potentially representing the cleavage reaction products; each NAG6 cross links two neighboring λ lysozyme molecules. Here we use NMR and MD simulations to study the interaction of λ lysozyme with the inhibitors NAG4 and NAG6 in solution. This allows us to study the interactions within the complex prior to cleavage of the polysaccharide. 1 HN and 15 N chemical shifts of λ lysozyme resonances were followed during NAG4/NAG6 titrations. The chemical shift changes were similar in the two titrations, consistent with sugars binding to the cleft between the upper and lower domains; the NMR data show no evidence for simultaneous binding of a NAG6 to two λ lysozyme molecules. Six 150 ns MD simulations of λ lysozyme in complex with NAG4 or NAG6 were performed starting from different conformations. The simulations with both NAG4 and NAG6 show stable binding of sugars across the D/E active site providing low energy models for the enzyme-inhibitor complexes. The MD simulations identify different binding subsites for the 5th and 6th sugars consistent with the NMR data. The structural information gained from the NMR experiments and MD simulations have been used to model the enzyme-peptidoglycan complex.

UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase from the snail Biomphalaria glabrata - structural reflections.

UDP-GalNAc:polypeptide GalNAc transferase (ppGalNAcT; EC 2.4.1.41) is the initiating enzyme for mucin-type O-glycosylation in animals. Members of this highly conserved glycosyltransferase family catalyse a single glycosidic linkage. They transfer an N-acetylgalactosamine (GalNAc) residue from an activated donor (UDP-GalNAc) to a serine or threonine of an acceptor polypeptide chain. A ppGalNAcT from the freshwater snail Biomphalaria glabrata is the only characterised member of this enzyme family from mollusc origin. In this work, we interpret previously published experimental characterization of this enzyme in the context of in silico models of the enzyme and its acceptor substrates. A homology model of the mollusc ppGalNAcT is created and various substrate peptides are modelled into the active site. We hypothesize about possible molecular interpretations of the available experimental data and offer potential explanations for observed substrate and cofactor specificity. Here, we review and synthesise the current knowledge of Bge-ppGalNAcT, supported by a molecular interpretation of the available data.

Structural Aspects of the O-glycosylation Linkage in Glycopeptides via MD Simulations and Comparison with NMR Experiments.

A powerful conformational searching and enhanced sampling simulation method, and unbiased molecular dynamics simulations have been used along with NMR spectroscopic observables to provide a detailed structural view of O-glycosylation. For four model systems, the force-field parameters can accurately predict experimental NMR observables (J couplings and NOE's). This enables us to derive conclusions based on the generated ensembles, in which O-glycosylation affects the peptide backbone conformation by forcing it towards to an extended conformation. An exception is described for ß-GalNAc-Thr where the α content is increased and stabilized via hydrogen bonding between the sugar and the peptide backbone, which was not observed in the rest of the studied systems. These observations might offer an explanation for the evolutionary preference of α-linked GalNAc glycosylation instead of a ß link.

Molecular Conformations of Di-, Tri-, and Tetra-α-(2→8)-Linked Sialic Acid from NMR Spectroscopy and MD Simulations.

By using molecular dynamics simulations with an efficient enhanced sampling technique and in combination with nuclear magnetic resonance (NMR) spectroscopy quantitative structural information on α -2,8-linked sialic acids is presented. We used a bottom-up approach to obtain a set of larger ensembles for tetra- and deca-sialic acid from model dimer and trimer systems that are in agreement with the available J-coupling constants and nuclear Overhauser effects. The molecular dynamic (MD) simulations with enhanced sampling are used to validate the force field used in this study for its further use. This empowered us to couple NMR observables in the MD framework via J-coupling and distance restraining simulations to obtain conformations that are supported by experimental data. We used these conformations in thermodynamic integration and one-step perturbation simulations to calculate the free-energy of suggested helical conformations. This study brings most of the available NMR experiments together and supplies information to resolve the conflict on the structures of poly- α -2,8-linked sialic acid.

Modeling of Oligosaccharides within Glycoproteins from Free-Energy Landscapes.

In spite of the abundance of glycoproteins in biological processes, relatively little three-dimensional structural data is available for glycan structures. Here, we study the structure and flexibility of the vast majority of mammalian oligosaccharides appearing in N- and O-glycosylated proteins using a bottom up approach. We report the conformational free-energy landscapes of all relevant glycosidic linkages as obtained from local elevation simulations and subsequent umbrella sampling. To the best of our knowledge, this represents the first complete conformational library for the construction of N- and O-glycan structures. Next, we systematically study the effect of neighboring residues, by extensively simulating all relevant trisaccharides and one tetrasaccharide. This allows for an unprecedented comparison of disaccharide linkages in large oligosaccharides. With a small number of exceptions, the conformational preferences in the larger structures are very similar as in the disaccharides. This, finally, allows us to suggest several efficient approaches to construct complete N- and O-glycans on glycoproteins, as exemplified on two relevant examples.

Explicit Representation of Cation-π Interactions in Force Fields with 1/r4 Nonbonded Terms.

The binding energies for cation-π complexation are underestimated by traditional fixed-charge force fields owing to their lack of explicit treatment of ion-induced dipole interactions. To address this deficiency, an explicit treatment of cation-π interactions has been introduced into the OPLS-AA force field. Following prior work with atomic cations, it is found that cation-π interactions can be handled efficiently by augmenting the usual 12-6 Lennard-Jones potentials with 1/r4 terms. Results are provided for prototypical complexes as well as protein-ligand systems of relevance for drug design. Alkali cation, ammonium, guanidinium, and tetramethylammonium were chosen for the representative cations, while benzene and six heteroaromatic molecules were used as the π systems. The required nonbonded parameters were fit to reproduce structure and interaction energies for gas-phase complexes from density functional theory (DFT) calculations at the ωB97X-D/6-311++G(d,p) level. The impact of the solvent was then examined by computing potentials of mean force (pmfs) in both aqueous and tetrahydrofuran (THF) solutions using the free-energy perturbation (FEP) theory. Further testing was carried out for two cases of strong and one case of weak cation-π interactions between druglike molecules and their protein hosts, namely, the JH2 domain of JAK2 kinase and macrophage migration inhibitory factor. FEP results reveal greater binding by 1.5-4.4 kcal/mol from the addition of the explicit cation-π contributions. Thus, in the absence of such treatment of cation-π interactions, errors for computed binding or inhibition constants of 101-103 are expected.

A speculation on the tandem fasciclin 1 repeat of FLA4 proteins in angiosperms.

The Arabidopsis thaliana Fasciclin like arabinogalactan protein 4 (FLA4) locus is required for normal root growth in a linear genetic pathway with the FEI1 and FEI2 loci coding for receptor-like kinases. The two Fas1 domains of FLA4 are conserved among angiosperms but only the C-terminal Fas1 domain is required for genetic function. We show that at low salt deletion of the N-terminal Fas1 domain of transgenic FLA4 leads to enhanced root elongation compared to the tandem Fas1 wild type version. Modeling the hypothetical interaction between FLA4 and FEI1 we show that the predicted interaction is predominantly involving the C-terminal Fas1 domain. Relative conformational mobility between the two FLA4 Fas1 domains might regulate the interaction with the FEI receptor kinases. We therefore speculate that the FLA4 FEI complex might be a sensor for environmental conditions in the apoplast.

Plasmon Field-Enhanced Fluorescence Energy Transfer for Hairpin Aptamer Assay Readout.

Surface plasmon field-enhanced fluorescence energy transfer is employed for sensitive optical readout of a reversible hairpin aptamer assay that is suitable for continuous monitoring of low-molecular-weight chemical analytes. A hairpin aptamer specific to adenosine and adenosine triphosphate with Alexa Fluor 647 fluorophore attached to its 5' end was anchored via 3' end thiol to a gold thin film. Molecular spacers were used to control the distance of the fluorophore from the surface in the aptamer "off" and "on" states. The specific binding of the target analyte changes the aptamer conformation, which alters the distance of the fluorophore from the gold surface and translates to variations in the detected fluorescence intensity. The plasmonically mediated fluorescence signal increases the measured signal-to-noise ratio and allows for real-time observation of the analyte binding. Theoretical as well as experimental study of the optical signal dependence on fluorophore orientation, design of spacers, and angular distribution of collected light is presented for rational design of the assay. The detected sensor signal increased by a factor as large as 23 upon switching the aptamer from the "off" to "on" state due to the hairpin opening associated with the specific capture of target analyte.

Recombinant plant-derived human IgE glycoproteomics.

The increasing biotechnological interest in human IgE antibodies demands advanced systems which allow their proper expression. However, this is still a challenge due to the complexity of the molecule, particularly regarding the diverse N-glycosylation pattern. Here, we present the expression of recombinant IgE in wild type and glycan-engineered Nicotiana benthamiana plants and in-depth N-glycosylation analyses. Mass spectrometric profiling revealed that plant IgE has a site occupancy rate that ranges from non-occupied at glycosite 6 (GS6) to 100% occupancy at GS1 and 2. Similarly to human cell-derived IgE, plant versions carry complex N-glycans at GS1-5 and oligomannosidic structures at GS7. Computational modelling suggests that spatial position (or orientation) of glycans can impair processing or site occupancy on adjacent glycosites. IgE expressed in glycoengineered and wild type plants carry, respectively, GnGn and plant-typical GnGnXF structures at large homogeneity. This contrasts with the glycan diversity of HEK cell-derived IgE, carrying at least 20 different glycoforms. Importantly, IgE glycoengineering allows the control of its glycosylation, a so far unmet need when using well-established expression systems. This enables the elucidation of possible carbohydrate-dependent IgE functions. SIGNIFICANCE: Targeted glycosylation of recombinant proteins may provide an advantage in therapeutic applications. Despite increasing biotechnological interest in IgE antibodies, knowledge and impact of glycosylation on this antibody class are scarce. With the ability to glyco-engineer recombinant IgE, we provide an important step towards the generation of IgE with other targeted N-glycans. This will facilitate detailed structure-function studies and may lead to the production of IgE with optimized activities.